1. Field of the Invention
This invention relates to a magnetooptical recording medium for recording/reproducing information using a laser beam utilizing a magnetooptical effect, and more particularly, to a magnetooptical recording medium on which high-density information can be recorded, and a method of reproducing the recorded information.
2. Description of the Related Art
In the field of rewritable high-density recording, magnetooptical recording media have attracted notice, in which information is recorded by writing magnetic domains on a magnetic thin film using the thermal energy of a semiconductor laser, and the information is read using a magnetooptical effect. Recently, there has been an increasing demand for large-capacity recording media by increasing the recording density of the magnetooptical recording media.
The track recording density of an optical disk, such as a magnetooptical disk or the like, greatly depends on the wavelength of the laser beam used in the reproducing optical system, and the numerical aperture of the objective lens used in the system. That is, when the wavelength .lambda. of the laser beam used in the reproducing optical system and the numerical aperture N of the objective lens are determined, the diameter of the beam waist is determined. Hence, the detectable limit of the spatial frequency in signal reproduction is about 2NA/.lambda..
Accordingly, in order to realize high-density recording in a conventional optical disk, the wavelength of the laser beam used in the reproducing optical system must be shortened, and the numerical aperture NA of the objective lens must be increased. However, there is a limitation in improving the wavelength of the laser beam and the numerical aperture of the objective lens. Accordingly, techniques have been developed, in which the recording density of a recording medium is increased by improving the configuration of the recording medium and the method of reading information.
For example, in Japanese Patent Laid-open Application (Kokai) No. 3-93058 (1991), a signal reproducing method is proposed, in which a signal is recorded in a recording holding layer in a multiple layers comprising a reproducing layer and the recording holding layer magnetically coupled with each other. After aligning the orientation of magnetization in the reproducing layer, the reproducing layer is heated by projecting a laser beam, the signal recorded in the recording holding layer is transferred to the region whose temperature has been raised, and the transferred signal is read.
According to this method, the region, whose temperature has been raised to a transfer temperature by being heated by the laser beam and from which a signal is detected, can be limited to a smaller region than the spot size of the reproducing laser beam. Hence, interference between codes during a reproducing operation is reduced, so that a signal having a period equal to or less than the diffraction limit of light can be reproduced.
However, in the magnetooptical reproducing method described in Japanese Patent Laid-open Application (Kokai) No. 3-93058 (1991), since the signal detection region which is effectively used is smaller than the spot size of the reproducing laser beam, the amplitude of the reproduced signal is greatly reduced, and therefore a sufficient reproduced signal cannot be obtained.
Moreover, since the magnetization of the reproducing layer must be aligned in one direction before projecting the laser beam, a magnet for initializing the reproducing layer must be added to a conventional apparatus. Hence, the above-described reproducing method has the problems that, for example, the magnetooptical recording apparatus has a complicated configuration, whereby the cost of the apparatus increases, and the size of the apparatus cannot be reduced.